1. Field of the Invention
The present invention relates to an engine for a motorcycle that transmits a starting power to a crank shaft from a starter motor via a power transmission mechanism and a one-way clutch.
2. Description of the Related Art
In an engine for a motorcycle provided with a starter apparatus, it is preferable that a one-way clutch is arranged on a crank shaft for reducing mechanical loss of starting power, and the one-way clutch is normally installed in an end portion of the crank shaft. Further, the one-way clutch is arranged at an outermost end in an axial length direction of the crank shaft in order to couple the one-way clutch to a starter motor arranged in an outer portion of a crank case so as to freely transmit a power.
Further, in the engine for the motorcycle, as a mechanism for transmitting rotation of the crank shaft to a cam shaft for intake and exhaust valves, there are a gear transmission mechanism including a plurality of gears, and a chain transmission mechanism including a sprocket and a chain. It is advantageous for the chain transmission mechanism to have few parts, be low in weight, and allow for freedom of layout.
FIG. 5 shows a conventional engine for a motorcycle provided with a chain transmission mechanism for driving a cam shaft and a starter apparatus (Japanese Unexamined Patent Publication No. 8-86223). FIG. 6 is a view as seen from an arrow VI in FIG. 5 showing a state in which a cover 115 for a starter apparatus and the like are detached. FIG. 7 is an enlarged view of a cross section of FIG. 6 taken along a line through respective axes. A crank case is omitted and the cover is partially shown by two dotted line.
In FIG. 5, a cam chain chamber 104 is formed in one end portion of a crank case 100 in an axial length direction of a crank shaft 105 and the cover 115 for the starter apparatus is attached thereto. One end portion of the crank shaft 105 extends to a starter chamber 106 within the cover 115 through the cam chain chamber 104, a cam drive sprocket 110 is formed in the crank shaft 105 within the cam chain chamber 104, and a one-way clutch 102 for the starter apparatus is attached to the crank shaft 105 portion within the starter chamber 106. A generator rotor 103a of a generator 103 is attached to the other end portion of the crank shaft 105. A cam chain 108 is wound between the cam drive sprocket 110 and a pair of cam drive sprockets 107 attached to cam shafts mounted on a cylinder head.
In FIG. 7, the one-way clutch 102 is constituted of a closed-end tubular clutch outer member 116 firmly fixed (spline fitted) to an end portion of the crank shaft 105, a clutch inner member (a clutch hub) 118 rotatably supported by a boss portion 116c of the clutch outer member 116 via a needle bearing 117, and a clutch element 119 such as a sprag or the like arranged between the clutch outer member 116 and the clutch inner member 118, and is structured so as to transmit power in an engine normal rotational direction only to the clutch outer member 116 from the clutch inner member 118.
A starter gear 120 is provided in the clutch inner member 118 of the one-way clutch 102, and the starter gear 120 is coupled to an output pinion 126 of a starter motor 125 via first and second idle gears 121 and 122 (a gear transmission mechanism) arranged within the cover 115 (FIG. 5) so that the power of the starter motor 125 is transmitted to the starter gear 120. The idle gears 121 and 122 are respectively provided with small-diameter tooth portions 121a and 122a and large-diameter tooth portions 121b and 122b, and are rotatably supported respectively by idler shafts 127 and 128. The starter gear 120 is engaged with the small-diameter tooth portion 121a of the first idle gear 121, the large-diameter tooth portion 121b of the first idle gear 121 is engaged with the small-diameter tooth portion 122a of the second idle gear 122, and the large-diameter tooth portion 122b of the second idle gear 122 is engaged with the output pinion (gear) 126 of the starter motor 125.
In FIG. 6, the idler shaft 127 of the first idle gear 121 engaging with the starter gear 120 is arranged in a rear side of an area (or range) surrounded by the cam chain 108 (a so-called cam chain line), the second idler shaft 128 is arranged in a rear upper side of the first idler shaft 127, and the starter motor 125 is arranged in a rear upper side of the second idler shaft 128.
In the case that the starter motor 125 is driven (rotated) at a time of starting the motorcycle engine mentioned above, the power of the starter motor 125 is transmitted to the starter gear 120 from the output pinion 126 via the second and first idle gears 122 and 121, and is transmitted to the crank shaft 105 from the starter gear 120 via the one-way clutch 102 in FIG. 5.
If the starter gear 120 on the crank shaft 105 is arranged in an inner side of the one-way clutch 102 in the axial length direction of the crank shaft 105 as shown in FIG. 7, there is generated a disadvantage that a diameter of the starter gear 120 is limited. In other words, due to the necessity of avoiding interference with a mating face 130 of a clutch cover for housing a clutch for connecting and disconnecting a wheel drive force for the motorcycle, a pulsar 112, a chain guide support portion 132 and the like in FIG. 6, a diameter of the starter gear 120 is limited. On the other hand, if it is intended to increase the diameter of the starter gear 120, an entirety of the crank case must be enlarged. Further, in FIG. 7, in the first idle gear 121, since the small-diameter tooth portion 121a is positioned in an inner side in the axial length direction of the crank shaft 105 rather than the clutch outer member 116, and since the large-diameter tooth portion 121b is positioned in an outer side in the crank axial length direction rather than the clutch outer member 116, a boss portion 121c having a fixed length in the axial length direction of the crank shaft 105 is necessary for coupling the large and small tooth portions 121b and 121a, and therefore, the first idle gear 121 is enlarged and the weight is increased.
Further, as shown in FIG. 6, in the structure in which the idler shaft 127 of the first idle gear 121 engaging with the starter gear 120 is arranged in the rear side (the outer side) of an area surrounded by the cam chain 108, due to the necessity of avoiding interference with the mating face 130 of the clutch cover and the chain guide 133, and the first idle gear 121, a center distance L1 between an axial core of the crank shaft 105 and an axial core of the first idler shaft 127 is elongated, a distance W1 from the axial core of the crank shaft 105 to the axial core of the output pinion 126 of the starter motor 125 is accordingly elongated, the starter gear 120 is enlarged and the starter apparatus itself is enlarged. Further, since the starter motor 125 having a great weight is positioned rearward apart from the crank shaft 105, it is possible to prevent a mass of the engine from being concentrated.
In this case, with respect to the starter apparatus as shown in FIGS. 5 to 7, the starter apparatus shown in FIG. 8 is structured such that the possibility of the interference mentioned above can be removed by making the diameter of the starter gear 120 smaller than the clutch outer member 116. However, if the diameter of the starter gear 120 is made smaller, it is necessary to enlarge a tooth face width t of the starter gear 120 to provide sufficient strength. Accordingly, the one-way clutch 102 protrudes to an outer side in the axial length direction of the crank shaft 105, and a bank angle θ is limited. Further, it is necessary to enlarge a tooth face width of the small-diameter tooth portion 121a of the first idle gear 121 in correspondence to the starter gear 120, and the weight thereof is increased. Further, since the small-diameter tooth portion 121a overlaps the clutch outer member 116 as seen from a side direction (a direction of an arrow A), it is necessary to avoid interference between an outer peripheral wall 116a of the clutch outer member 116 and the small-diameter tooth portion 121a of the first idle gear 121 in an axial length direction of the crank shaft 105, and therefore, assembly is more complicated.